Cathode follower output circuit



June 9' 3965 F. e. VON KUMMER fi fl CATHODE FOLLOWER OUTPUT CIRCUIT Filed June 20, 1955 FIG. I

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Pl P2 CURRENT 0| t: +2 :3 :4 cs *5 t7 m INVENTOR F.G. VON KUMMER ATTORNEY United States Patent 3,192,479 CATHODE FDLLOWER OUTPUT CIRCUIT Ferdinand G. von Kummer, Norwalk, Conn., assignor to Sperry Rand Corporation, a corporation of Delaware Filed June 20, 1955, Ser. No. 516,375 16 Claims. (Cl. 328-58) Tlris invention relates to output circuits of the cathode follower type commonly used to drive low impedance loads, especially those capacitive in nature.

More specifically, the invention relates to circuitry for improving the response of the cathode follower for comparatively large signals.

Cathode follower output circuits known to the art operate satisfactorily if the amplitude of the applied signal is kept within certain limits. If the applied signal goes beyond the cut-off range of the tube "or the tube cuts off during the negative going portion of a wave or pulse, the signal will be markedly distorted. During the time the tube is cut off, the impedance of the cathode follower circuit becomes high and approaches the value of the cathode resistor.

Where the load is capacitive and the signal applied to the cathode follower is in the form of pulses, the negative going portion of the pulses will be highly dis torted.

The recovery time of these negative going pulses can be reduced by decreasing the value of the cathode resistor of the cathode follower stage, which requires tubes of higher ratings, or by reducing the capacity of the load by the use of properly terminated low impedance lines.

Where there are many circuits involved, as for example in a computer, these alternatives result in either increased cost, power consumption, heat dissipation, or a combination of these factors.

The present invention provides means of decreasing the impedance of the output load only when it is required, that is, during the time of discharge of the effective capacitance of the load. In the present invention, this is accomplished without an increase in the number of tubes required, the ratings of the tubes, the use of specially terminated low impedance transmission lines, or a decrease in the value of cathode resistor.

The invention also results in extending the frequency range over which the cathode follower will operate for a prescribed acceptable amount of distortion with large signal inputs. In this invention, a discharge path is provided, in parallel with the cathode follower load through the amplifier stage driving the cathode follower, which provides a low discharge impedance path only at the time when the signal drives the cathode follower negative. The circuit driving the cathode follower is modified by addition of a biasing resistor and a diode, and is further described by reference to the attached drawings, which illustrate an embodiment of the invention, in which:

FIG. 1 is a schematic diagram of the invention; and

FIG. 2 shows the wave forms obtained by the use of the invention in solid lines. conventional circuitry are shown in dotted lines.

Referring to FIG. 1, V is a conventional amplifier stage which has been modified, as will be described hereinafter, followed by V which is similarly arranged as a conventional cathode follower stage. The grid bias resistors and cathode circuits are shown as returned to ground. G-rid bias for the cathode follower V is provided by resistor R in a conventional manner. The output of the cathode follower is connected across R to the load as shown.

In accordance with the invention, the anode of the rectifier G is connected to the cathode of V and the cathode connected between resistors R and R in the output circuit of V Wave forms obtained by G may be any unidirectional element, such as vacuum tube diode, silicon diode, germanium diode, selenium rectifier, or the like. A germanium or silicon type diode is preferred because of its characteristic high for-ward conductance and low interelectrode capacitance.

The value of resistor R is selected so that the steady state voltage drop across R is approximately the same as the steady state bias voltage of the cathode follower.

The operation of the circuit shown in FIG. 1, will be described in connection with the waveforms, shown in FIG. 2, which are of the character used in digital computers. The negative pulse P shown in FIG. 2, line a, is applied to the input of the amplifier stag-e V where it is amplified and inverted. This output signal on the plate of V is shown on line b of FIG. 2 as P The output of V is then applied to the grid of V The output of V is across the resistor R and is shown as P of line 0, FIG. 2.

The next succeeding pulse P follows at time t and is similarly shown in lines a, b and c of FIG. 2.

Line at shows the pulses P and P in dotted form that would be obtained at the output of amplifier V with an unmodified amplifier cathode follower circuit, which does not incorporate the principles of this invention. Line 2 shows the same pulses as they would appear at cathode of follower V again Without modification. In a conventional circuit the leading edge of P (line e) at time t would be faithfully reproduced since the impedance of the cathode follower is low. However, at time t the pulse P line b, goes negative with an amplitude and slope such that V cuts off and thereby removes the relatively low output discharge impedance desired for the capacitive load. The energy stored in the capacitor must, therefore, discharge through R which now offers a high impedance thereby increasing the discharge time. The timerequired for the trailing edge of the pulse to return to the original reference level is shown in line e of FIG. 2 to now be from t to t The long discharge time required for the trailing edge limits the frequency of the pulse rate to be applied tothe circuit.

The foregoing disadvantages are overcome by the addition of the rectifier G and the bias resistor R as shown, which permits the energy stored in the load to be discharged at a faster rate, such as shown by pulse P of line 0 in FIG. 2. The rectifier G and the resistor R form a conductive path in parallel with the load R and cooperate with the other circuit elements in the following manner.

When a positive signal is applied to the grid of V the cathode of G is positive with respect to its anode, and no current flows in the circuit. Since the impedance of the cathode follower stage is low for positive signals, no additional conductive path is required at this time.

When a negative signal or a negative going signal is applied to the grid of V the cathode of G becomes negative with respect to its anode and the diode conducts permitting current to flow through the amplifier circuit. At this time the capacitiveload becomes the source of potential and R ris the load resistance for V which is at Zero bias.

Referring to FIGS. 1 and 2, it is seen that at the time t when the negative going portion of the pulse P line b, is applied to V the pulse on the input of V is positive going and driving the tube V into greater conduction, thereby lowering its impedance offered to the current flowing in the G R circuit.

The additional discharge path lessens the time required for the negative going pulse to return to its original reference level, as seen by reference to FIG. 2, line c.

This permits the circuit to be operated at a higher fre- Patented June 29, 1965 t quency and in a digital computer decreases the time of computation.

Current flowing in the Ci -R circuit is represented by line of FIG. 2 from which it is apparent that the load discharge current flows only during the negative going portion of the pulse.

The value of resistor R is selected such that the steady state DC. voltage drop across it is approximately equal to the bias voltage of the cathode follower tube. This condition maintains a state of equilibrium such that little or no appreciable current flows through the G R circuit.

The value of R is not critical. Should there be a reasonable difference in the D.C. voltage drops across the rectifier, the current flowing in the circuit does not impair the operation of the circuit.

In the embodiment shown, a current of approximately 1 milliampere flows in the G R circuit during steady state condiitons. This current flow is represented on line 1 of the FIG. 2 by the amplitude of the waveform between the time intervals and t Even though the embodiment shown has been described in connection with pulses similar to those encountered in computers, the circuit operates in the same manner and to the same advantage with sinusoidal and other wave forms as well, to permit the cathode follower to be driven harder and still result in more faithful wave shapes without the use of additional tubes or increased current consumption.

Although only a preferred embodiment of the invention has been described with capacitive loads, it is understood that the invention described herein can be used with other types of loads where a high impedance is matched to a low impedance load.

Changes and modifications may be made from the described embodiment without departing from the scope of the invention as set forth in the appended claims,

What is claimed is:

1. An electronic output circuit for driving a capacitive load with a signal comprising a first electron discharge tube having an anode, cathode and control grid, a resistor connected in the cathode circuit of said first tube, said capacitive load being connected acrosssaid cathode resistor, a second electron discharge tube having an anode, cathode and control grid, a first resistor connected to the anode of said second tube, a second resistor connected to said first anode resistor and the anode supply of said second tube, coupling means connecting the anode of said second tube to the grid of said first tube, and unidirectional conductive means connected between said first and second resistors in the anode circuit of said second tube and the cathode of said first tube to provide a conductive path through said second tube when said first tube is cut off by a negative going signal.

2. Apparatus same as claim 1 whereinsaid conductive path means consists of a rectifier having its anode connected to the cathode of said first tube, and its cathode connected between said first and second resistor in the anode circuit of said second tube.

3. Apparatus same as claim 2 wherein the value of said first resistor is such that the steady state voltage drop across said resistor will be approximately equal to the steady state bias voltage of said first tube. 4. In an electronic output circuit for matching high impedance circuits to a low impedance load, comprising an amplifier stage driving a cathode follower stage, each stage having an output and input, a unidirectional discharge circuit connected from the output of said cathode follower to the output of said amplifier stage whereby aconductive. path is provided through said amplifier stage when the impedance of the cathode follower stage be-. comes high due to the presence of a large negative going signal on the input of said cathode follower stage.

5. The invention of claim 4 wherein the conductive path includes a unidirectional element and a biasing resistor.

6. Apparatus same as .claim 5 wherein said amplifier stage comprises a thermionic device having at least an anode, cathode and grid, said bias resistor being connected to the anode of said device, and a second resistor connected between said bias resistor and a source of positive potential, the output of said amplifier stage being taken from the junction of said bias resistor and said plate, said unidirectional conductive path being connected from the output of said cathode follower stage to the junction of said bias resistor and said second resistor, the circuit parameters being chosen so that said conductive path is operative only when the cathode follower stage is cut off due to a negative going signal applied to its input.

7. In an electronic circuit consisting of an amplifier stage coupled to a cathode follower stage driving a load, the invention of which is characterized by the provision of a unidirectional conductive path from the load through the amplifier circuit whenever a negative going signal drives the cathode follower circuit to cutoff.

8. An electronic circuit for driving a capacitive load in response to an input signal comprising an amplifier stage and a cathode follower stage each stage having an input and an output, the output of said amplifier stage being connected to the input of the cathode follower stage, said cathode follower stage including a thermionic device having at least a cathode, anode and grid, the output of said cathode follower stage being taken directly from the cathode of said cathode follower thermionic device, and a unidirectional path from the output of said cathode tollower thermionic device to the output of said amplifier stage adapted to provide a low impedance conduction path through said amplifier stage when the impedance of said cathode follower stage is high.

9. The apparatus of claim 8 wherein said unidirectional conductive path includes a unidirectional element and a biasing resistor.

10. The apparatus of claim 9 wherein said amplifier stage comprises a thermionic device having at least an anode, cathode and grid, 21 source of positive potential connected to said anode, a first resistor connected to said potential source and said bias resistor connected between said first resistor and said anode.

11. The apparatus of claim 10 wherein the input to said grid of said cathode follower, thermionic device is applied thereto directly from said anode of said amplifier stage thermionic device across a grid bias resistor, whereby the steady state voltage drop across the bias resistor in said unidirectional discharge path will be approximately equal to the steady state bias voltage of said cathode follower 5 age.

12. A pulse amplifier circuit comprising a first amplifier having an input circuit and a high-impedance anode output circuit; a second amplifier having an input circuit and a low-impedance cathode output circuit, the input circuit of said second amplifier being connected to the output circuit of said first amplifier; and a diode, said diode being coupled between said cathode output circuit and said anode output circuit; said diode being poled so as to be conductive when signals in said cathode output circuit are more positive than said anode output circuit, whereby when signals of widely varying dynamic range are applied to the input circuit of said first amplifier, they appear in substantially undistorted greatly amplified form in said cathode output circuit.

13. The method of achieving in combination the advantages of the low impedance output of the cathode follower and the high gain characteristic of the anode-loaded amplifier, respectively, in a pulse amplifier, comprising the steps of: applying signal pulses to the input circuit of an anode-loaded amplifier; amplifying the said signal pulses; deriving therefrom an amplified signal-in-a-high-impedance; applying said amplified signal to the input of a cathode-follower amplifier; deriving therefrom a corresponding signal-in-a-low-impedance; conducting that portion of said signal-in-a-low-impedance which is more positive than the signal-in-a-high-impedance back to said input of a cathode-follower amplifier whereby the cathode-follower output signal is maintained to follow exactly the said signal applied thereto.

14. A pulse amplifier circuit comprising in combination a cathode-follower amplifier having at least a grid, a plate and a cathode; a diode; and .a plate-loaded amplifier having at least a grid, a plate, and a cathode; the grid of said cathode follower and the plate of said plate-loaded amplifier being connected together, the junction of said grid and said plate being connected to a source of positive potential through a first load device and further connected through a passive network to ground, the cathode of said cathode follower being connected to an output terminal, the plate of said cathode follower being connected to said source of positive potential, the cathode of said plate-loaded amplifier being connected to a ground terminal, the grid of said plate-loaded amplifier being connected to an input terminal, said diode being connected between said output terminal and the said junction, whereby signals applied to said input terminal are amplified over a wide dynamic range with a high degree of accuracy and reproduction fidelity.

15. A system for delivering rapidly terminating pulses to a capacitive load comprising a driver stage and a cathode follower output stage, each stage including an electronic discharge tube having .a cathode, a control grid, and an anode, a resistor connected between the anode of the driver stage and a source of positive potential, a low impedance connection between the anode of the output stage and a source of positive potential, said cathode of said driver stage being directly connected to ground, there being a capacitive load connected between the cathode load terminal of the output stage and ground, a coupling connection for applying control pulse waveforms to the control grid of the driver stage, a low impedance connection between the anode of the driver stage and the control grid of the output stage, a diode connected in conducting relation between the cathode load terminal of the output stage and the anode of the driver stage, and a passive network connected between the anode of said driver stage and ground whereby the diode and said passive network form a series circuit connected in shunt with the capacitive load.

16. A pulsing system for delivering a rapidly decaying pulse to a high resistance capacitive load comprising a controlled driver stage, a grounded cathode in said driver stage, a cathode follower output stage, a cathode output terminalin said output stage, said output terminal being directly connected to the capacitive load, a control grid in said driver stage, a control grid in said output stage, a low impedance connection from the anode of the driver stage to the control grid of the output stage whereby said output stage is fully conducting when said driver stage is non-conducting and means for rapidly terminating current flow through said output stage in response to conduction in said driver stage, said means including a diode connected in conducting relation from the cathode output terminal of said output stage to the anode of the driver stage whereby the load capacitance is discharged through the diode and the driver stage.

References Cited by the Examiner UNITED STATES PATENTS 2,005,27 9 6/35 Van Geel et a1 317233 2,489,272 11/49 Daniels 330-68 2,588,427 3 5 2 Stringfield. 1 2,708,240 5/55 Casey. 2,744,169 5/56 Deming. 2,821,626 1/5 8 Freedman.

FOREIGN PATENTS 696,441 9/53 Great Britain.

GEORGE N. WESTBY, Primary Examiner.

ARTHUR GAUSS, SIMON YAFFEE, HERMAN KARL SAALBACH, HARRY GAUSS, Examiners. 

1. AN ELECTRONIC OUTPUT CIRCUIT FOR DRIVING A CAPACITIVE LOAD WITH A SIGNAL COMPRISING A FIRST ELECTRON DISCHARGE TUBE HAVING AN ANODE, CATHODE AND CONTROL GRID, A RESISTOR CONNECTED IN THE CATHODE CIRCUIT OF SAID FIRST TUBE, SAID CAPACITIVE LOAD BEIG CONNECTED ACROSS SAID CATHODE RESISTOR, A SECOND ELECTRON DISCHARGE TUBE HAVING AN ANODE, CATHODE AND CONTROL GRID, A FIRST RESISTOR CONNECTED TO THE ANODE OF SAID SECOND TUBE, A SECOND RESISTOR CONNECTED TO SAID FIRST ANODE RESISTOR AND THE ANODE SUPPLY OF SAID SECOND TUBE, COUPLING MEANS CONNECTING THE ANODE OF SAID SECOND TUBE TO THE GRID OF SAID FIRST TUBE, AND UNIDIRECTIONAL CONDUCTIVE MEANS CONNECTED BETWEEN SAID FIRST AND SECOND RESISTORS IN THE ANODE CIRCUIT OF SAID SECOND TUBE AND THE CATHODE OF SAID FIRST TUBE TO PROVIDE A CONDUCTIVE PATH THROUGH SAID SECOND TUBE WHEN SAID FIRST TUBE IS CUT OFF BY A NEGATIVE GOING SIGNAL. 